The present invention relates to an apparatus and method of cryoablation, and more particularly for cryoablation using multiple probes introduced into the body of a patient through a common introducer, so as to perform cryoablation of a particular volume of tissue while minimizing damage to adjacent healthy tissues.
A variety of medical conditions are preferentially treated by ablation of tissues within the body. Classically, ablation was performed using invasive surgical procedures requiring cutting or destroying tissues between the exterior of the body and the particular site whose ablation is desired. More recently, less invasive procedures have been developed, which bring about the destruction of selected tissues using a probe or probes which penetrate to the area to be operated, and destroy the selected tissue by transferring energy to those tissues; RF energy, light (laser) energy, microwave energy, and high-frequency ultra-sound energy are among the forms which have been used. However all such methods have the common disadvantage that while transferring energy to the tissues whose destruction is intended, they tend also to transfer energy, through conduction, convection, and other natural processes, to nearby healthy tissues as well. All such energy transfer methods ultimately result in heat release, causing complications and adverse effects. Noticeable pain results, the functioning of nearby healthy tissues is impaired, and the healthy tissues are often damaged or destroyed. Moreover, in some cases tissues exposed to thermal energy or other forms of energy that raise their temperatures secrete substances that may be toxic to adjacent healthy tissues.
In contrast, cryoablation provides a number of important advantages over other ablation techniques. Cryoablation provides better control of the ablated volume than is attainable using other procedures. Moreover, real-time imaging during cryoablation, using ultrasound and MRI techniques, is helpful and straightforward, since the frozen tissue is clearly seen under these imaging techniques. Also, cryoablation, unlike heat radiation techniques, allows for repeatable and/or complementary treatment of the affected area. Cryoablation is considered to cause less pain to the patients. Some scientific evidence supports the conclusion that there is less morbidity and less risk of mortality as a result of cryoablation procedure compared to other minimally invasive and traditional techniques. For these and other reasons, cryoablation has recently become a popular method for certain types of minimally invasive ablation procedures. Examples include the treatment of prostate malignant tumors and of benign prostate hyperplasia (BPH), and the creation of trans-myocardial channels to effect trans-myocardial revascularization.
Yet, cryoablation procedures also have an inherent disadvantage. Cryoprobes when activated typically form at their tip what is know in the art as an “ice ball”, a volume which is frozen by exposure to the low temperatures developed by the cryoprobe. Unfortunately, the radius of the volume in which total destruction of tissues is achieved (such destruction of tissues being the purpose of the operation) is typically only half of the radius of the volume within which tissues are more or less severely damaged. Since the volume of a sphere is proportional to the cube of the radius, the volume of total cell destruction, for a particular ice-ball, will typically be only the order of one-eighth of the volume of the area that is frozen during the operation and more or less severely damaged. The disadvantage is clear: if a single ice-ball is used to destroy a selected volume, and the ice-ball is large enough to ensure the complete destruction of that volume (which complete destruction would be desired in the case of a malignancy, for example), then a surrounding volume approximately seven times larger will be more or less severely damaged. That surrounding volume will typically include much healthy tissue that would preferably be left healthy and intact. In the case of ablation of the prostate, for example, freezing of surrounding tissues using simple cryosurgical techniques will typically damage or destroy, and create temporary or permanent impairment of the function of, the prostatic urethra, the anus, and various bundles of nerves in the prostatic area.
One method of solving this problem is taught by U.S. Pat. No. 6,142,991 to Schatzberger, teaching the use of a series of ice-balls of small dimensions, such as can be created by a two-dimensional array of cryoprobes whose depth of penetration can be measured and controlled, so as to achieve accurate three-dimensional placement of a plurality of ice-balls, in a manner that conforms to the dimensions and form and placement of the lesion to be destroyed. In other words, Schatzberger's apparatus defines a volume of controllable form and dimension, for cryoablation. The ice-balls created by the apparatus are each of small dimensions, and they are placed so as to be contiguous to one another or to overlap each other. This arrangement results in a reduction of the amount of tissue that is damaged but not destroyed, and permits more accurate definition of the exact form and dimensions of the ablated tissue.
The mechanism described by Schatzberger is not, however, well adapted to every application of cryoablation. It is relatively complex, and requires penetration of the affected area by a multiplicity of individually introduced and individually handled cryoprobes. It could not be used, for example, in the context of cryoablating benign prostate hyperplasia (BPH) through the urethra, a relatively non-invasive treatment method described in U.S. patent application Ser. No. 09/301,576, filed Apr. 29, 1999, and incorporated herein by reference. That procedure requires an apparatus which is both simpler and more compact than that described by Schatzberger, in that the procedure requires the operating portion of the cryogenic apparatus to be introduced to the area of the lesion by means of a cystoscope, in order to reduce reducing trauma to healthy tissue.
Thus there is a widely recognized need for, and it would be highly advantageous to have, a method and apparatus for cryoablation that provides for the destruction of a defined volume of tissue, yet which minimizes damage to adjacent tissues. It would be further advantageous to have a method of cryoablation using an apparatus that creates such an extended volume of cryoablation yet is contained within a single introducer. It would be yet further advantageous to have such an introducer which could be introduced through an operating channel of a catheter or cystoscope, enabling it to reach the proximity of the region to be treated with a minimum of trauma to intervening tissues.
Referring now to another aspect of prior art, two-stage heating and cooling has successfully been used in surgical cryoablation systems, particularly in two-stage cooling of a high-pressure gas used to achieve cryogenic temperatures using Joule-Thomson heat exchangers. Two-stage cooling presents the advantages of more rapid and more efficient cooling than would be possible in a single Joule-Thomson cooling stage. In U.S. Pat. No. 5,993,444 to Ammar a cryogenic probe utilizes two stages of Joule-Thomson cooling to achieve low temperatures at the operating end of the probe. Ammar describes, however, a single probe so cooled.
Schatzberger, in the patent previously cited, describes two-stage cooling in a multi-probe system. In FIG. 6a Schatzberger teaches a plurality of cryosurgical probes connected by flexible connectors to a common housing which includes a pre-cooling element for pre-cooling the high-pressure gas flowing to the probes, this element being preferably a Joule-Thomson heat exchanger used as a cooler. Schatzberger's system thus utilizes two-stage cooling, with pre-cooling taking place extracorporeally in the housing and a second cooling stage taking place in each individual cryoprobe. Furthermore, the mechanism Schatzberger describes has the disadvantage that the pre-cooled gases must be transported a considerable distance between the housing and the probe, and the conduit connecting the probe to the housing, which must remain flexible, must also be thermally insulated.
Consequently, it would be further advantageous to have a cryoablation apparatus and method which enables the pre-cooling of a plurality of cryoprobes within a single introducer, such that the pre-cooling stage of a two-stage Joule-Thomson heat exchange process can take place in close proximity to a second stage of cooling which takes place within the individual cryoprobes.